The present invention relates to gas ionization apparatus and methods generally and more specifically to capacitance coupled gas plasma reactors and methods.
Capacitance coupled plasma reactors are usually constructed with a pair of parallel plate electrodes facing each other, spaced apart in parallel, and placed inside a vacuum chamber. An external electric field, either DC or AC, is applied to the opposite electrodes. Under low pressure and with proper spacing between the electrodes, a stable plasma can be generated and maintained by first ionizing and then creating a glow discharge in gas flowing between the electrodes. Multiple pairs of alternating polarity parallel plates can be spaced apart and/or stacked together to form multiple regions where plasma discharge may occur. Such capacitance coupled plasma reactors have been widely used in a variety of industries for applications such as substrate etching, substrate cleaning, substrate film deposition, gas treatment, ion beam source and for various chemical reactions.
As the term xe2x80x9ccapacitance coupled plasmaxe2x80x9d implies, the electrodes form a capacitor, typically of the parallel plate type. The most fundamental type is simply two flat plates of opposite electrical polarity and is often referred to as a xe2x80x9cplanar diode.xe2x80x9d The electrodes may be arranged in a variety of geometric configurations, including configurations having curved surfaces, such as concentric parallel cylinders or concentric spheres with parallel tangents. Typically the surfaces of the alternate polarity electrodes will be equally spaced throughout the structure to maintain the parallel plate relationship. The geometric regularity and symmetry between the surfaces of the electrodes in such structures are thought to be desirable for the production of a uniform electric field and hence a more uniform plasma. Concave or convex pairs of flat plate electrodes have also been used to focus or defocus the intensity of the plasma concentration in specific regions for special applications such as focus sputtering, focus etching or to provide a focused ion source. A number of prior art capacitively coupled parallel plate electrode designs having different geometric configurations are taught in U.S. Pat. No. 4,735,633, entitled Method and System for Vapor Extraction From Gases, which issued to the inventor of the present invention and is assigned to the assignee of the present invention. The electrode configurations taught in the ""633 patent provide large surface area to volume ratio for compact plasma reactors. Reactors employing electrode configurations as taught in the ""633 patent have been successfully used in industry to provide greater than 99% reaction efficiency.
In addition to electrode spacing, another critical parameter for plasma generation and maintenance in capacitively coupled plasma reactors is the operating pressure. A stable glow discharge plasma can be more efficiently and easily maintained at lower pressures. This is because the production and maintenance of the plasma depends on the ionization of gas molecules in the reactor volume to produce sufficient secondary electrons to participate in the cascade collisional ionization process to offset and balance the loss of electrons (and ions) to the electrode surfaces. The mean free path, i.e., the average distance a primary electron will travel in the reactor volume before colliding with a molecule to create secondary electrons, depends upon the operating pressure. Generally, the higher the pressure, the smaller the value of the mean free path. The value of the mean free path places a limitation on the distance in which primary electrons within the electric field potential between the electrodes can accelerate to acquire the ionization potential energy needed to facilitate the ionization process. Thus, the smaller the value of the mean free path, the less ionization potential energy an electron will acquire for a given operating potential before colliding with a gas molecule, and the less secondary ionization is likely to occur.
For a given operating pressure, the electrode spacing determines the number of mean free path ionization collisions an electron will be involved in before it reaches and is lost to the electrode surface. For very short electrode spacing, no glow discharge can be generated and maintained. This space is known as dark space. Once a plasma is ignited in the reactor volume, it becomes a conducting sheet itself equivalent to an electrode. Between the plasma and the electrodes, there is always a space gap in which glow discharge ionization does not occur. Only ions and electrons are accelerated in this gap without further glow ionization discharge, and such space is the known as the xe2x80x9cdark space shield.xe2x80x9d The thickness of the dark space shield is also pressure dependent.
Thus, the point at which the gas molecules will break down and a stable glow discharge plasma can be generated and maintained depends on the relationship of the applied external electric field potential, the breakdown voltage, the electrode spacing and the operating pressure. Paschen experimentally found that the breakdown potential voltage (V) varies with the product of pressure P (in units of Torr) and the electrode spacing d (in units of cm). The relationships Pashen identified are known as the law of glow discharge and are reflected in the xe2x80x9cPaschen curvesxe2x80x9d shown in FIG. 1. FIG. 1 shows Paschen curves 10 for several different gases. The electrode design for a capacitively coupled, parallel plate plasma reactor must adhere to the physical requirements shown by the Paschen curves.
The Paschen curves 10 of FIG. 1 show there is a minimum breakdown voltage (V) for every gas for the product of Pd at about 1 Torr-cm, i.e., at about point 15. Thus, in practical terms, if the spacing between parallel plate electrodes is fixed at about 1 cm, the lowest external voltage necessary to apply to the electrodes to initiate ionization and breakdown of a gas under vacuum is obtained at a pressure of about 1 Torr. As can be seen from the Paschen curves 10, for a given electrode spacing d, as the pressure P increases, the minimum external voltage necessary to satisfy the 1 Torr-cm breakdown parameter slowly increases. However, as pressure is reduced, the minimum necessary voltage sharply increases (in linear scale of Pd). Thus, for example, given a power supply that can provide a maximum voltage of 1000 V, a reactor with fixed electrode spacing of about 1 cm can be operated at pressures up to about 300 Torr for neon gas, for example for neon light applications. But the same 1000V power supply will not be capable of generating and maintaining a plasma in Neon gas at pressures below about 0.1 Torr unless the electrode spacing is increased several times, such that the breakdown voltage 15 of the Paschen curve 10 occurs at a Pd value below the 1000V maximum supply limit.
Thus, in practical application, the relationships shown in the Paschen curves 10 determine the minimum electrode spacing and hence the minimum size for a reactor for a given power supply rating and operating pressure range. In most applications, it is desirable to use a low voltage power supply, either AC or DC, rather than a high voltage power supply because of the intrinsically lower cost of lower voltage supplies. It is also desirable to use smaller spacing between electrodes so that the reactor will be smaller and more compact. However, when operating at pressures below about 0.5 Torr, which may be required in certain applications such as in many semiconductor processing applications, it is a must to increase the electrode spacing to a few centimeters or more, thus increasing the reactor size, or alternatively to employ considerably more expensive high voltage power supplies. Though additional magnetic field sources could be used to confine the plasma in very low pressure operation application, this solution is very costly, further complicates and upsets the capacitive coupling of the applied and dissipated plasma energy, and introduces more side effects.
The aforementioned ""633 patent teaches to maximize the efficiency of a reactor of a given size by maximizing the surface area of the electrodes within the reactor volume in a specific way to increase the reaction efficiency. Although the reactor taught in the ""633 patent was primarily intended for use in semiconductor fabrication applications to break down and dispose of noxious exhaust gases, the plasma processing described in the patent also provides a very efficient means to process materials, such as by sputtering, etching, deposition, surface treatment, etc. It also provides an efficient gaseous chemical reaction means to produce desirable byproducts, for example chemical synthesis, polymer formation, chemical dissociation, etc. Advantages of this type of plasma processing over other chemical methods include substantially reduced energy consumption and substantially improved reaction efficiency at relatively low temperatures. One plasma reactor of the type taught in the ""633 patent that has been used commercially is trademarked DryScrub(copyright) and is sold by the assignee of the present invention. As taught in the ""633 patent, the DryScrub(copyright) reactor takes advantage of a large electrode surface area to plasma volume ratio and a long gas flow path to maximize chemical reaction on the electrode surfaces. This maximizes the reaction rate and reaction efficiency compared to gas phase reaction in the gas stream itself.
Thus, as taught in the ""633 patent, for a pair of parallel plate electrodes, the area of the face of each surface of each electrode is A, and the total surface area of the opposing faces of the pair of electrodes is 2A. The volume enclosed between the faces is 2Ad for a fixed spacing d between electrodes. For low-pressure operation, the electrode spacing d must be increased for the reasons previously described. The plasma volume also increases with an increase in the electrode spacing d, and therefore the surface area to volume ratio decreases inversely proportional to increased spacing d. Thus, a decrease in operating pressure will result in the loss of some or all of the surface reaction advantages unless the surface area of the electrodes can somehow be increased. Of course, one way to increase the surface area of the electrodes is to increase the size of the reactor and hence the electrodes. However, for various reasons, including cost, as well as application or design constraints, this may not be desirable or even feasible. Therefore, it is necessary to find a way to further increase the surface area of the electrodes within the reactor volume without increasing the size of the reactor for low pressure applications, among other things.
The present invention addresses this problem by providing a new and unique electrode design. A primary objective of the new electrode design is to substantially increase the surface area of the electrodes without substantially increasing the volumetric size of the reactor. The new electrode design provides highly efficient electrode surface reactions over a significantly broadened range of operating parameters in capacitively coupled parallel plate plasma reactors and methods of the type taught in the ""633 patent without any significant increase in size of the reactor. As such, the new electrode design also greatly increases the range of applications for such reactors and methods.
The conventional thinking, as demonstrated in the ""633 patent, has been to configure pairs of parallel plate electrodes with opposing faces extending laterally and without any intrusions into the open volume between the opposing or adjacent surfaces of the electrodes. It has been thought undesirable to have any surface portions extending or intruding into the space between the electrodes because that would reduce the distance between the electrodes at that point or those points. There has been a significant fear that this would create a short circuit path that would cause arcing between the electrodes. Thus, it has been thought desirable to design the electrodes so that their opposing surfaces should be as flat as possible and as smoothly curved as possible to avoid this perceived problem. Moreover, for the reasons discussed previously with respect to the Paschen curves, there has been a fear that reducing the spacing between electrodes could adversely impact the generation, maintenance and quality of the glow discharge plasma. This has been the conventional wisdom in the design and construction of electrodes.
The present invention contradicts the conventional wisdom relating to electrode design and construction. In the present invention, a pair of alternating polarity electrodes are configured with a plurality of xe2x80x9cL-shapedxe2x80x9d and xe2x80x9c7-shapedxe2x80x9d fin protrusions to form a so-called xe2x80x9cL7xe2x80x9d electrode structure. The protrusions extend into the open space between the adjacent opposing electrodes and are arranged in interleaved fashion. The electrodes with their interleaved protrusions form roughly square xe2x80x9cL7xe2x80x9d shaped channels with one or more spaces or gaps, for example at one or more diagonal corners. A more expansive embodiment employs a grid type design wherein multiple pairs of opposite polarity electrodes, each having protrusions, are stacked together such that the protrusions are interleaved in the space between the electrodes. The xe2x80x9cL7xe2x80x9d shape with the interleaved protrusions maintains the electrode spacing d between the parallel opposing faces of electrode pairs, but increases the electrode surface area within a given volume by four times or more. Numerous protrusion/fin geometries are possible including continuously curved surfaces or xe2x80x9cWxe2x80x9d shaped surfaces, which can provide even more surface area per unit of volume.
A plama reactor/generator apparatus and method embodying the xe2x80x9cL7xe2x80x9d electrode design of the invention includes a reactor body with an open interior volume. The reactor body includes a gas inlet and a gas outlet. The electrode apparatus is preferably configured as a unitary assembly that can be inserted into and removed from the interior volume of the reactor as a unit. The electrode assembly is typically electrically insulated from the reactor body. In an open system, the electrode assembly is enclosed within the interior of the reactor body and defines a plurality of subdivided gas flow paths between the gas inlet and gas outlet. In a static or closed system, the electrode assembly subdivides or partitions the gas volume into a plurality of cells, which may be according to a desired pattern. A power source in electrical communication with the electrode assembly generates a voltage potential between pairs of adjacent opposite polarity electrodes sufficient to ignite and maintain a plasma in a selected gas to be processed in the reactor. In one aspect of the invention, a flow of gas is introduced into the gas inlet at selected pressure, flow rate, and temperature and traverses the subdivided flow paths adjacent the surfaces of the electrodes to the gas outlet. A plasma having multiple distinct regions, preferably in at least partial communication, is generated and results in a highly efficient and complete chemical reaction of the gas upon the surfaces of the electrodes, the reactor thus carrying out a selected process on or employing the gas. In another aspect of the invention, the plasma generator is a closed system containing a gas. The plasma is formed in the gas, for example to generate luminescence. In this aspect, the invention is a plasma generator.